This invention relates to a process for producing small or minute capsules containing a water-immiscible material which comprises dissolving polymethylene polyphenylisocyanate in said water-immiscible material, the material to be encapsulated, dispersing the resulting mixture in an aqueous phase containing an emulsifier selected from the group consisting of the salts of lignin sulfonate and thereafter adding a polyfunctional amine, whereby the amine reacts with polymethylene polyphenylisocyanate to form oil-insoluble polyurea microcapsule walls about the water-immiscible material at the oil/water interface. The capsules may be produced to any desired size, for example, of the order of 1 micron up to 100 microns or larger, preferably the size of the microcapsules will range from about 1 to about 50 microns in diameter.
Capsules of this character have a variety of uses, as for containing dyes, inks, chemical reagents, pharmaceuticals, flavoring materials, pesticides, herbicides and the like. Any liquid, oil, meltable solid or solvent soluble material into which polymethylene polyphenylisocyanate can be dissolved and which is non-reactive with said isocyanate, may be encapsulated with this process. Once encapsulated, the liquid or other form is preserved until it is released by some means or instrumentality that breaks, crushes, melts, dissolves, or otherwise removes the capsule skin or until release by diffusion is effected under suitable conditions. The process of the invention is particularly suitable for the production of herbicide containing microcapsules of very small particle size, suspended in an aqueous solution.
Aqueous dispersions of pesticide and herbicide microcapsules are particularly useful in controlled release pesticide and herbicidal formulations because they can be diluted with water or liquid fertilizer and sprayed using conventional equipment, thereby producing uniform field coverage of the pesticide or herbicide. Additives such as film forming agents can be added directly to the finished formulation to improve the adhesion of microcapsules to foliage. In some cases, reduced toxicity and extended activity of encapsulated herbicides and pesticides have been noted.
A variety of techniques have heretofore been used or proposed for encapsulation purposes. In one such process, known as "simple coacervation", a polymer separates from a solvent solution of the polymer by the action of a precipitating agent that reduces the solubility of the polymer in the solvent (e.g., a salt or a non-solvent for the polymer). Patents describing such processes and their shell wall material include U.S. Pat. Nos. 2,800,458 (hydrophilic colloids); 3,069,370 and 3,116,216 (polymeric zein); 3,137,631 (denatured proteins); 3,418,250 (hydrophobic thermoplastic resins); and others.
Another method involves microencapsulation based on in situ interfacial condensation polymerization. British Pat. No. 1,371,179 discloses a process which consists of dispersing an organic pesticide phase containing a polymethylene polyphenylisocyanate or toluene diisocyanate monomer into an aqueous phase. The wall forming reaction is initiated by heating the batch to an elevated temperature at which point the isocyanate monomers are hydrolyzed at the interface to form amines, which in turn react with unhydrolyzed isocyanate monomers to form the polyurea microcapsule wall. One difficulty with this method is the possiblity of continued reaction of monomer after packaging. Unless all monomer is reacted during the preparation, there will be continued hydrolysis of the isocyanate monomer with evolution of CO.sub.2, resulting in the development of pressure when the formulation is packaged.
Various methods of encapsulation by interfacial condensation between direct-acting, complimentary reactions are known. Within these methods are reactions for producing various types of polymers as the capsule walls. Many of such reactions to produce the coating substance occur between an amine, which must be of at least difunctional character and a second reactant intermediate, which for producing a polyurea is a difunctional or polyfunctional isocyanate. The amines chiefly used or proposed in these methods are typified by ethylene diamine, having at least 2 primary amino groups. U.S. Pat. No. 3,577,515 is illustrative of encapsulation by interfacial condensation.
U.S. Pat. No. 3,577,515 describes a continuous or batch method which requires a first reactant and a second reactant complimentary to the first reactant, with each reactant in separate phases, such that the first and second reactants react at the interface between the droplets to form encapsulated droplets. The process is applicable to a large variety of polycondensation reactions, i.e., to many different pairs of reactants capable of interfacial condensation from respective carrier liquids to yield solid film at the liquid interface. The resulting capsule skin may be produced as a polyamide, polysulfonamide, polyester, polycarbonate, polyurethane, polyurea or mixtures of reactants in one or both phases so as to yield corresponding condensation copolymers. The reference describes the formation of a polyurea skin when diamines or polyamines (e.g., ethylene diamine, phenylene diamine, toluene diamine, hexamethylene diamine and the like) are present in the water phase and diisocyanates or polyisocyanates (e.g., toluene diisocyanate, hexamethylene diisocyanate and polymethylene polyphenylisocyanate) are present in the organic/oil phase. In the practice of U.S. Pat. No. 3,577,515, the liquid which preponderates becomes the continuous phase liquid. That is, in forming oil containing microcapsules, the aqueous liquid would preponderate; when water encapsulated microcapsules are formed, the oil phase would preponderate.
Although a number of methods are available in the art for producing microencapsulated pesticide and herbicide formulations there are various disadvantages associated with the prior art methods. The encapsulated materials formed by the in situ interfacial polymerization process of British Pat. No. 1,371,179, require post-treatment to prevent continued carbon dioxide evolution and excessive caking, thereby increasing the costs of the finished product. For many processes of encapsulation, it is oftentimes necessary to separate the encapsulated material from the forming media. During the separation process, the capsule wall is subjected to great stresses and strains which can result in premature rupture of the capsules with concomitant loss of encapsulated material. These efforts also fall short of practical value in various other respects. Various experiments have indicated the difficulty in establishing the desired capsules in discreet form and avoiding coalescence of the partially formed capsules into a heterogenous mass of materials lacking distinct capsule formation. Control of capsule uniformity is troublesome in the prior art method. Very low concentrations of intended product relative to the total mixture are often obtained.
The present invention provides a new and improved encapsulation process which is rapid and effective and which avoids the necessity of separation of the encapsulated material from the continuous phase material. The present invention also eliminates the need for using a strong solvent in the organic phase resulting in a savings of energy, and packaging and equipment ware. In addition, direct combination of water-based herbicide and pesticide formulations are possible with other water-based pesticides.
The critical feature of the present invention resides in the use of lignin sulfonate emulsifiers, in particular, the salts of lignin sulfonate, as for example, the sodium, potassium, magnesium, calcium or ammonium salts, to achieve emulsions wherein a concentrated amount of water-immiscible material is present in the water-immiscible phase. Generally there will be greater than 480 grams per liter of water-immiscible material present. By use of the particular emulsifiers described herein, it is possible to retain the finished microcapsules in the original aqueous solution, thus avoiding the additional step of separation of the microcapsules from the original aqueous environment. Further, the finished microcapsules do not agglomerate nor does the aqueous capsule mass solidify when stored for extended periods of time or when exposed for short-terms to elevated temperatures.
The present invention is particularly advantageous when employed to encapsulate herbicides, especially the acetanilide and thiocarbamate herbicides like alachlor, butachlor, propachlor, triallate, diallate and the like. Experiments indicate that conventional oil/water herbicide emulsifiers fail to produce sufficiently stable emulsions to attain microencapsulation of concentrated amounts of herbicide materials and avoiding solidification of the oil/water mass when amine is added. Additionally, attempts to encapsulate concentrated amounts of acetanilide and thiocarbamate herbicides (four to five pounds per gallon) using traditional interfacial polymerization techniques, as for example that disclosed in U.S. Pat. No. 3,577,515, have resulted in unsatisfactory formulations because of the problem of herbicide crystal growth, as well as agglomeration or solidification of the finished suspensions. It is thought that herbicide crystal growth results from either incomplete encapsulation of the herbicidal material or from the passage of small amounts of herbicide through the polymeric shell wall. The problem is particularly acute with the acetanilide herbicides.
Crystal growth is very undesirable because once it occurs, the final formulations cannot be used directly; rather the microcapsules must be separated from the aqueous solution and resuspended in water before they can be sprayed in conventional agricultural herbicide and fertilizer spraying apparatus.
It is accordingly a particular object of this invention to provide a process whereby greater than 480 grams of acetanilide herbicides, e.g., alachlor, propachlor, butachlor and the like and thiocarbamate herbicides, e.g., triallate, diallate and the like, per liter of aqueous material, is encapsulated in a polyurea shell wall with the finished microcapsules being suspended in the original aqueous solution. The suspended microcapsules may be stored for extended periods of time and may be exposed for short-terms to elevated temperatures without the occurrence of agglomeration or solidification of the aqueous, capsule mass or herbicide crystal formation.